The XP-42 featured cooling fan in most, if not all of if it's iterations. The version with short prop shaft is depicted
here. It is possible that big spinners need fans to help out with cooling? Fans don't work for free, the BMW 801 used around 3.5% of power for the fan.
One (or more?) iteration of the powerplants of the XP-42 included the individual exhaust stacks in very much Fw 190 vogue, tested in winter of 1941/42.
Allies didn't needed Faber to make it's mistake in order to came out with a much better installation for their engines.
How hard is it to add another row of cylinders to the Nakajima Sakae?
Could the Homare be developed in the late 1930s?
https://en.wikipedia.org/wiki/Nakajima_Homare
A million dollar question

Sakae was already with two rows of cylinders.
The 1st two-row radial by Nakajima that I'm aware of is the Ha-5 from 1937. Power 890 HP at 4700 m, 950 HP for take off, 1260 mm diameter, 625 kg. Later developed into Ha-41 that powered eg. Ki-44, 1260 HP at 3700m, 1260 HP for take off, more RPM allowed, just 5 kg heavier, produced from 1940. 1st flight of the Ki 44 was in August 1940, or almost a year and a half after the Zero.
Ha-109 was a further development of the Ha-41, featuring a bigger superchager, with two-speed gearing and neccesarry strengthening. Power at 5200 m was 1220 HP, at 2100 M was 1440, and 1500 HP for take off. Weight 720 kg.
Our Zero with Ha-5/Ha-41/Ha-109 would've been a compettitive machine.
But to your question - let's say Nakajima develops an 18 cyl engine with same bore and stroke as the Sakae, on same technological level. That would've give around 1280 HP in 1st versions, and 1400+ in mid war.